1. Field of the Invention
The present invention relates to drug-eluting medical devices; more particularly, this invention relates to processes for crimping a stent to a delivery balloon.
2. Background of the Invention
A known stent retention process consists of three operations, stent crimp, split mold and stent press. Stent crimp is the process by which the stent is placed on the catheter. First the stent is placed on a pre-crimp mandrel and placed in the crimp machine. The crimp machine closes onto the stent applying radial force causing the stent's diameter to be reduced to that of the pre-crimp mandrel. The pre-crimp mandrel diameter is selected based on the delivery systems folded balloon profile. After pre-crimp, the stent is removed from the mandrel and placed on a balloon delivery system, balloon catheter, or delivery system. The balloon catheter is placed into the crimp machine and radial force applied to reduce the diameter of the stent onto the balloon catheter. No heat or catheter inflation pressure is applied during this operation.
The delivery system now with the stent in place on the balloon is sent to the split mold operation. The split mold operation applies heat and pressurizes the delivery system for a specified amount of time to a specified increased diameter causing the balloon to “pillow” between the stent struts, which further increases mechanical interaction between balloon and stent. The delivery system is then moved to a stent press where the stent and balloon are placed into a press machine and radial force again applied to reduce the profile to a specified diameter, thereby again increasing the mechanical interaction between balloon and stent.
Related devices are mentioned in U.S. Pat. No. 7,763,198 ('198 patent) which is commonly owned with the present application. An example of the “split mold” is depicted in FIGS. 1-3 of the '198 patent. The bore of the split mold is machined within the block that forms the body of the mold, with the two halves of the mold in place together during the machining. The diameter of the mold bore (FIG. 12) may be slightly larger than the outer diameter of a crimped stent on the balloon of the balloon catheter, or matched to that diameter, so that the stent does not radially expand during the stent mounting. The balloon is heated by heating the mold with via a conductive heating element member in the form of metal platens.
With the balloon catheter in position within the bore of the split mold, the mold is heated to an elevated temperature sufficient to soften the balloon but lower than the thermal limit of the drug disposed on or in the stent. The mold is heated to a temperature of about 160° F. to about 190° F., with the balloon catheter therein during the stent mounting procedure, to soften a balloon formed of polymeric material. The '198 patent shows a transverse cross section of the balloon catheter with stent gaps partially filled by balloon material so that the balloon material contacts and partially encapsulates the side surfaces of the stent struts, to mount the stent on the balloon.
According to the '198 patent, during the initial, or pre-crimping and/or re-crimping process, i.e., before and after, respectively, the split-mold process the balloon may be pressurized and heated to increase the protrusion of balloon material into the openings in the stent pattern, thereby further increasing stent retention on the balloon. The balloon may be pressurized in the range of 10 to 300 pounds per square inch (psi). The balloon may be heated to the range of about 70 degrees to 250 degrees Fahrenheit (21 to 121 degrees Celsius) during re-crimping. The mounted stent can be heated to about 130 degrees Fahrenheit (54 degrees Celsius) during re-crimping. The balloon may be pressurized to about 70 psi.
The '198 patent discloses various combinations of crimping, balloon pressure and heating of stent and balloon to reduce the stent profile and increase the retention of stent to balloon. However, the pre-crimp, split-mold and re-crimp phases of the process are performed separately using separate machines. This process is time-consuming and does not yield an optimal combination of profile and dislodgment or retention force of the balloon and stent. Dislodgment or retention force means the force needed to pull or dislodge the stent from the balloon. Further details on the meaning of dislodgment force or stent retention force may be found in U.S. application Ser. No. 11/938,127. What is needed is a process that simplifies the process of crimping a stent to a balloon, while also achieving the desired crimped stent profile and increasing the dislodgment force. Accordingly, there is a continuing need to improve upon the crimping methods for stents.